This invention relates to processes for constructing metal parts, and more particularly to processes for producing shaped metal parts having a low carbon content. In the prior art, various attempts were made to form shaped metal parts by injection molding. However, these attempted processes were found to be not suitable for making metal parts with a carbon content of less than 1.0%.
In comparison, injection molding works fine and is widely used to form ceramic parts. Basically, the first step for forming ceramic parts by an injection molding process consists of forming a mixture of a powdered ceramic, a binder material, a plasticizer, and an oil. The ceramic is used in a powdered form because then it can be easily shaped in a mold; the binder material functions as an adhesive which holds the particles of the ceramic powder together; the plasticizer functions to make the mixture more moldable; and the lubricant function to provide an easy release of the part from the mold.
After the powdered ceramic, binder material, plasticizers, and mold lubricant have been mixed to a substantially uniform consistency, the mixture is put into a mold of a predetermined shape. Various injection molding apparatus are used to fill the mold. These include for example, apparatus wherein the mixture is forced through a nozzle by a plunger into the mold. Other apparatus force the mixture into the mold through a barrel containing a screw which rotates and carries the material into the mold.
Subsequently, the green part is removed from the mold. Then the plasticizer is removed from the green molded part. Typically, a solvent which attacks the plasticizer, but does not substantially affect the binder material is utilized to accomplish this removal. This leaves the part with sufficient porosity such that when it is sintered the binder material may be driven out.
It is, of course, desirable to form metal parts by a similar process. This is because the process is carried out at temperatures which are substantially less than those required to form metal parts from molten metal. For example, temperatures of less than 300.degree. F. are typically required. A problem however, with carrying out the above described steps with metal powder being substituted for the ceramic powder is that the resulting parts exhibit a relatively high carbon content. This occurs bacause during sintering, the binder material that is in the green part decomposes and combines with the metal powder. In comparison, ceramic materials are relatively inactive; and thus they do not combine with the binder as it decomposes.
In order to lower the carbon content of the resulting metal parts, attempts have been made to increase the relative amount of plasticizer material and/or decrease the relative amount of binder material. The problem with this approach however, is that as the ratio of plasticizer material to binder material increases, the viscosity of the mixture decreases; and to produce metal structures with less than 1% carbon content with this process, the viscosity decreases past the point where the molded part has sufficient green strength to prevent distortion prior to sintering. This problem is most severe when the desired part has a complex shape.
Accordingly, it is one object of the invention to provide an improved method for making parts.
Another object of the invention is to provide a method of making shaped metal parts having less than 1.0% carbon content by injection molding. Still another object of the invention is to provide a method of making metal parts from a mixture containing at least two substantially different binder materials, some of which can be selectively removed prior to sintering.